1 Field of the Invention
The present invention relates to DC motors. More particularly, but not by way of limitation, the present invention relates to a DC motor having a motor housing and a controller housed in the housing.
2. Background of the Invention
DC motors are well known in the art and are manufactured according to one of several constructions, i.e., permanent magnet type motors, wound-field type motors, brushless type motors, etc. Regardless of the type, when speed or torque control is required, a controller must be coupled with the motor. Presently, with permanent magnet or wound-field type motors, a separate controller is used. Normally such a controller provides a pulse width modulated output to vary the RMS voltage applied to the motor.
Trolling motors are also well known in the art and, most often, employ a permanent magnet DC motor. Generally speaking, a trolling motor is a relatively small electric motor coupled to a propeller for propelling a boat, or other water craft, at a relatively low speed. Typically, the electric motor and propeller are positioned at the bottom of a support column rotatably supported by a bracket which is attached to the boat. Traditionally, a control head located at the top of the support column houses electrical circuitry for controlling the motor. In addition, the control head may also house a steering motor and associated circuitry to provide rotation of the support column to steer of the boat.
Tolling motors are available in a variety of configurations, for example: variable speed versus discrete speed settings; automatic steering versus manual steering; a reversing controller versus a non-reversing controller, etc. In addition, manufacturers typically offer motors of different torque ratings, optional methods for controlling the motor, i.e., a hand control versus a foot pedal, and different operating voltages. Thus it can be seen that a trolling motor manufacturer wishing to appeal to a broad range of purchasers will likely have to produce a large number of individual products. Maximizing the interchangeability of components among various models can significantly reduce both recurring and nonrecurring costs, as well as significantly reduce the time and cost of bringing new models to market.
Additional concerns arise over trolling motors which use a pulse width modulated (PWM) speed control. In such motors, a solid state switch such as a bipolar transistor, MOSFET, IGBT, or the like, is switched at a variable duty cycle to control the voltage applied to the motor, and hence the speed of the motor. As is the case with any such solid state switch handling non-trivial amounts of electrical current, the dissipation of heat becomes a concern. In one school of thought, the solid state switch is placed in the control head with the control circuitry. While this location may facilitate maintenance when switches fail and may also reduce concerns during switching transients, as opposed to alternate locations, the opportunities to dissipate heat, and thus, maintain a relatively cool solid state switch, are rather poor. Typically the solid state switch is mounted on a heat sink with exposure to the environment. In such a placement, air movement around the control head is relied on to carry away the heat. Another limitation to this placement if that relatively long wires carry the electrical current from the switch to the motor. At typical PWM frequencies, this raises the potential for significant levels of electromagnetic emissions which my interfere with other on-board systems.
In another school of thought, the solid state switch is located in the motor housing which is submerged during operation of the motor. Typically, the motor housing is aluminum and the switch is in physical contact with the interior of the housing. With the housing submerged, water will be in contact with the exterior of the housing and provide a heat sink which is virtually infinite. As is well known in the art, the theoretical life of a transistor is inversely related to its operating temperature. Thus, while the transistor is much more difficult to access for repair when located in the motor housing, it should theoretically last longer, reducing the need for access anyway. Unfortunately, placing the switch remote from the controller creates its own set of problems, particularly during switching transients, such as ringing, increased rise and fall times, etc. Another advantage of locating the transistor in the motor housing is that only the gate drive, or base drive, signal is sent between the control head and the motor which dramatically reduces the switched current flowing through the conductors in the support column. Unfortunately, even this signal creates a potential for electromagnetic emissions.
Analogous problems exist with DC motors used in other applications. In general, external PWM controllers emit significant levels of electromagnetic interference, at least in part due to the wires between the controller and the motor. In addition to increased noise, the length of wiring between a controller and its motor can effect switching transients.
Furthermore, heat sinking of switching components of the controller is almost always required. Since heat produced by such components must be dissipated in the environment, controller heat sinks are often placed on an exterior surface, or other area with good air flow. On the other hand, most motor housings have a relatively large external surface area over which heat could be dissipated if heat producing components were located in the housing.
It is thus an object of the present invention to provide a DC motor which provides the advantages of placing the solid state switch in proximity of the PWM controller while providing the heat sink capabilities and electromagnetic emission levels associated with locating the solid state switch in the motor housing.
It is a further object of the present invention to provide a trolling motor which provides the advantages of placing the switching components of the controller proximate the controller circuitry while providing the heat sink capabilities and electromagnetic emission levels associated with locating such components in the trolling motor housing.
It is still a further object of the present invention to provide a trolling motor controller which allows modular assembly of various models of trolling motors to reduce the number of individual inventory items required to produce such models.